How to import 12-bit tif image sequence in Video Editor?
According to the Blender Manual, 12-bit TIFF is not supported (but 8 and 16 bit TIFF is): https://docs.blender.org/manual/en/dev/data_system/files/media/image_formats.html

I suggest converting the sequence to a different format (e.g. 16-bit TIFF, which is supported); there are several softwares that can achieve this. (I often use FFmpeg for this purpose—please let me know if you'd like help with FFmpeg.)